1. Field of Invention
The present invention pertains to the field of control systems. More particularly, this invention relates to providing a standard interface to process control devices which are adapted to differing field-bus protocols.
2. Art Background
Industrial process control systems typically make use of a variety of devices such as temperature sensors, pressure sensors, and actuators. Such devices are hereinafter referred to as process control devices. In addition, industrial process control systems commonly make use of field-level control buses which are specialized for the process control environment. Such specialized buses include Profi-bus, LonTalk, SDS, and DeviceNet buses to name a few. Communication buses specialized for the processes control environment are hereinafter referred to as field-buses.
Typically, one or more centralized controllers in such an industrial process control system are used to manage the process control devices. Such a centralized controller may be referred to as a programmable logic controller (PLC). A PLC is usually connected to one or more process control devices via one of the differing types of field-buses. A PLC typically includes a processor that executes a set of application code for communicating with and controlling the corresponding process control devices.
Typically, the application code in a PLC and the process control devices managed by the PLC are adapted to the communication protocol of the particular field-bus being employed. For example, a PLC that connects to process control devices via a Profi-bus typically executes application code that is adapted to the Profi-bus communication protocol. In addition, the process control devices connected to the Profi-bus are usually adapted to the communication protocol of the Profi-bus. Similarly, PLCs and process control devices connected together via an SDS bus are typically adapted to the SDS bus protocol.
It is often desirable to integrate differing process control subsystems into a larger system. Such a situation may arise when two process control plants that use differing field-buses are merged. For example, one plant may have an existing Profi-bus based process control system while the other may have an existing SDS based process control system. It is usually desirable to provide a mechanism for communication between the two differing systems rather than perform an expensive retrofit of process control devices.
One prior solution for providing communication between systems having differing protocols can be found in the field of computer networks in which protocol gateways are commonly used to translate between differing network protocols. For example, protocol gateways are commonly used to provide protocol translation between Ethernet networks and token-ring networks. Such protocol gateways are typically well suited for communication between computer networks which merely exchange files and data.
In contrast, a typical application program running on a PLC in a process control system performs real-time sampling and control of sensors and actuators in a precise manner in which many dependancies among the process control devices can exist. The specialized protocols of field-buses have evolved to meet these specialized needs. As a consequence, a protocol gateway between differing field-buses would likely be very complex and probably could not provide complete translation between protocols. Application programs in a system that included such protocol gateways would probably have to handle at least a portion of all of the field-bus protocols in use in the control system regardless of the existence of protocol gateways.
For example, an application program for a system that included a Profi-bus connected to an SDS bus through a protocol gateway would typically have to handle both Profi-bus and SDS protocols to some extent. Moreover, if the system were to include a LonTalk bus with a corresponding protocol gateway then the application program would probably have to handle all three protocols to some extent. The complexities worsen as more and more differing types of field buses are integrated into the system through protocol gateways. Unfortunately, such complexities typically increase the time and cost of developing application programs in the system and increase the difficulty and cost of maintaining the process control system.
In addition, providers of process control devices are confronted with the task of producing devices that are capable of communication over each of the wide variety of field-bus types in use. For example, the manufacturer of a particular temperature sensor designed for the SDS bus would usually develop permutations of the sensor adapted for the Profi-Bus and SDS buses if they wish to sell into industrial environments that use those types field-buses. Unfortunately, such design permutations typically increases manufacturing and development costs and maintenance costs of such process control devices.